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The Mystery of The R-Pod Total Frontal Area

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Happy Tripping View Drop Down
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Post Options Post Options   Quote Happy Tripping Quote  Post ReplyReply Direct Link To This Post Topic: The Mystery of The R-Pod Total Frontal Area
    Posted: 12 May 2019 at 7:00pm

Ultralight trailers like the R-Pod are touted as being able to be towed by smaller vehicles such as vans and small SUVs. That is their big selling point, a big tow vehicle is not required. Enter - Wind Resistance.

Wind resistance is very important for travel trailers. The more that there is, the greater is the stress on the powertrain and wheels of the tow vehicle and the lower the gas mileage is. If the wind is from the front, the speed of the trailer adds to the wind resistance, if from behind, it decreases it. Additionally, wind resistance at slow speeds like 45 mph is relatively minor, however as speeds are increased, it increases much faster than the actual increase in speed. Going from 55 mph to 75 mph is a 35% increase but wind resistance, and the strain on the powertrain, increases about 250%. (People with ‘L’ or ‘M’ rated tires take note) . Obviously, with all these variables, ‘wind resistance’ is not a static number.

The wind resistance of a travel trailer is roughly proportional to the ‘Total Frontal Area’ but protruding things like fenders, rooftop a/c units or awning mounts, and the underside of the trailer, also have effects that increase the effective total frontal area. To me, the published Total Frontal Areas of tow vehicles for travel trailers are a subject of great mystery and seem to be nearly worthless. However, this doesn’t bother the tow vehicle product liability lawyers. Using a complicated formula that is based on variables that are not apparently standard throughout the industry and that also are apparently not publicized, they have established required Total Frontal Areas for many if not all tow vehicles. These seem to exclude smaller vehicles.

Cynic that I am, I think that the lawyers have jumped in to give their companies the greatest protection possible in their powertrain warranties. The poor travel trailer manufacturers in response have carefully neglected to announce their trailer’s frontal areas in order not to hurt their sales. The R-Pod’s total frontal area is reported to be maybe around 50 sq ft. So, just for fun, please look in your operator’s manual and let us know if your tow vehicle passes or fails its manufacturers’s total frontal area requirements. I know that my entirely satisfactory Jeep Cherokee (towing capacity 4500 lbs) fails, and I am very interested to see who passes.

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jalong View Drop Down
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Post Options Post Options   Quote jalong Quote  Post ReplyReply Direct Link To This Post Posted: 12 May 2019 at 7:27pm
2014 Jeep Grand Cherokee maximum trailer frontal area 55 sq ft for all drive trains
John & Sue
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offgrid View Drop Down
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Post Options Post Options   Quote offgrid Quote  Post ReplyReply Direct Link To This Post Posted: 12 May 2019 at 8:11pm
I agree that the front area A by itself is essentially useless. The important parameter is really CdA or drag coefficient x frontal area. The Cd of rpods is pretty good. And yes, drag force increases with the square of the relative wind speed so the power requirement to overcome drag increases with the cube of the wind speed (because power = force x distance traveled per unit time). 

Total power requirement is the sum of the rolling resistance and the air drag. Since rolling resistance is relative small at freeway speeds and doesn't vary much, its fairly easy to roughly back into the air drag component based on your fuel consumption if you have that at a couple of different speeds. I did that and got a CdA of about 16 sq ft for my rig, which accounts for about 2/3 of the total horsepower requirement at 60 mph. I estimated A at 45 sq ft which gives a Cd of around 0.35, which is not bad.  A boxier trailer would be worse of course. 

For the record there is no published frontal area towing limit for my 2012 Highlander. If there was one I'd ignore it anyway, for the reasons we've stated. If your vehicle doesn't consume fuel excessively, overheat, or run at a high rpm towing at freeway speeds then air drag is not your limiting factor, weight and weight balance will be much more important to keep track of. 

Here's a calculator if you want to play around with it. 

2015 Rpod 179
2012 Toyota Highlander
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podwerkz View Drop Down
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Post Options Post Options   Quote podwerkz Quote  Post ReplyReply Direct Link To This Post Posted: 12 May 2019 at 9:47pm
Of course the towing vehicle creates a bit of a vacuum and/or vortices behind it, depending on the shape and size of it. This probably helps reduce trailer drag in calm winds, or slight headwinds, but broadside winds get to have an effect on the entire rig, more or less.

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offgrid View Drop Down
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Post Options Post Options   Quote offgrid Quote  Post ReplyReply Direct Link To This Post Posted: 13 May 2019 at 6:04am
Good point.  The drag of the rig is what matters, not the drag of the trailer separately. Depending on the aerodynamics of the tow vehicle the addition of the trailer might have a more or less dramatic increase on the combined drag. And if you want you can add aero devices like the Purpleline product to the TV, to try to reduce it. 

The numbers I worked back to in my post were for my combined rig. YMMV (literally). 


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tcj View Drop Down
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Post Options Post Options   Quote tcj Quote  Post ReplyReply Direct Link To This Post Posted: 13 May 2019 at 8:30am
Originally posted by Happy Tripping

So, just for fun, please look in your operator’s manual and let us know if your tow vehicle passes or fails its manufacturers’s total frontal area requirements. I know that my entirely satisfactory Jeep Cherokee (towing capacity 4500 lbs) fails, and I am very interested to see who passes.

2014 Toyota Tacoma Access cab V6 4x4 with towing package.  Trailer frontal area is not even mentioned in the owners manual.
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john in idaho View Drop Down
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Post Options Post Options   Quote john in idaho Quote  Post ReplyReply Direct Link To This Post Posted: 13 May 2019 at 8:59am
Seems like I saw a post a few years ago on this forum.  Someone added a canoe to the top of the pickup topper and increased gas mileage a few points.  but my memory may be faulty.
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lostagain View Drop Down
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Post Options Post Options   Quote lostagain Quote  Post ReplyReply Direct Link To This Post Posted: 13 May 2019 at 9:00am
I think the design of the Pod is so sleek and appealing that it magically resists air molecules as though it glides on clouds in a near weightless environment.  The only improvement, which I've previously proposed, would be some fin like wings coming off the back of the trailer at about the level of the spoiler.  Once installed, it would be like pulling the trailer in the vacuum of space.  Confused
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offgrid View Drop Down
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Post Options Post Options   Quote offgrid Quote  Post ReplyReply Direct Link To This Post Posted: 13 May 2019 at 9:16am
Originally posted by john in idaho

Seems like I saw a post a few years ago on this forum.  Someone added a canoe to the top of the pickup topper and increased gas mileage a few points.  but my memory may be faulty.

i have a kayak I was thinking of taking camping, maybe I'll try it. Another way to determine air drag is through coast down testing. All you need is a couple of miles of dead flat smooth road, no wind, no traffic, and a smart phone to video your speedometer during the tests.  
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lostagain View Drop Down
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Post Options Post Options   Quote lostagain Quote  Post ReplyReply Direct Link To This Post Posted: 13 May 2019 at 9:22am
.... and if you put the wings on the back edge of the Pod, it'll easily double the coasting distance.  Ermm
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Fred & Maria Kearney
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